Fuel vapor storage canisters are by now a standard feature of production automobiles in the United States. The basic design includes an elongated canister housing with closed ends that is substantially filled with a bed of packed fuel adsorbent granules, typically activated carbon. Nylon is a material choice for the canister housing, because of its durability and light weight. However, nylon and similar materials are subject to expansion both from water absorption and heat. Canisters are typically mounted underhood, an environment that is highly subject both to high heat and water splash. The carbon bed is usually retained in the canister housing by at least one plug or end plate that is pressed into the end face of the carbon bed to keep it tightly packed. The end plate is sized based on the nominal size of the perimeter of the inner surface of the canister housing.
Under the hood, environmentally induced expansion or swelling of the canister housing can potentially cause two problems. One is that the carbon bed becomes loose, since it does not expand in volume correspondingly. This can be compensated by making the end plate movable, and biasing it continually into the face of the carbon bed so that it remains tightly packed. A more serious problem is that canister housing expansion can create a significant increase in the perimeter of the inner surface, causing it to pull away from the edge of the end plate. Granules of the carbon bed can fall through, reducing the canister efficiency and capacity, and potentially interfering with the compression spring.